An industrial robot is typically formed as an arm comprising a number of joints, where a joint has a movable element being moved by an actuator, such as a motor connected to a gear, in relation to a stationary element. The movable element may thereby be moveable in relation to the stationary element, such as through an angular or linear movement.
One important feature of a robot is the accuracy of the control. The movements of the various joints of a robot have to be made within well defined tolerances. In order to do this it is necessary to calibrate at least one of the joints from time to time with a well-defined calibration position. Calibration may especially be needed after a maintenance operation, for instance after an exchange of a motor or measurement system.
In order to perform the calibration, the calibration position must then first be found.
In the prior art there are some schemes used for detecting positions of robot joints.
U.S. Pat. No. 5,418,890 discloses a system where an angle is determined when a round bar comes into contact with a round hole.
JPS 60-211505 discloses a sensor that determines an angle based on detecting the change of diameter of a rotating wheel.
A similar idea is disclosed in U.S. Pat. No. 7,566,198. Here sensors are used for detecting changes in the exterior of a rotary member for determining homing angles.
However, there is in view of these documents still room for improvement. The present invention is therefore directed towards the problem of finding a calibration position to be used for calibrating a robot joint.